xfrm6: Properly handle unsupported protocols

We don't catch the case if an unsupported protocol is submitted
to the xfrm6 protocol handlers, this can lead to NULL pointer
dereferences. Fix this by adding the appropriate checks.

Fixes: 7e14ea15 ("xfrm6: Add IPsec protocol multiplexer")
Signed-off-by: Steffen Klassert <[e-mail]>

net/ipv6/xfrm6_protocol.c

···
 {
 	int ret;
 	struct xfrm6_protocol *handler;
+	struct xfrm6_protocol __rcu **head = proto_handlers(protocol);
+
+	if (!head)
+		return 0;
 
 	for_each_protocol_rcu(*proto_handlers(protocol), handler)
 		if ((ret = handler->cb_handler(skb, err)) <= 0)
···
 	struct xfrm6_protocol __rcu **pprev;
 	struct xfrm6_protocol *t;
 	bool add_netproto = false;
-
 	int ret = -EEXIST;
 	int priority = handler->priority;
+
+	if (!proto_handlers(protocol) || !netproto(protocol))
+		return -EINVAL;
 
 	mutex_lock(&xfrm6_protocol_mutex);
 
···
 	struct xfrm6_protocol __rcu **pprev;
 	struct xfrm6_protocol *t;
 	int ret = -ENOENT;
+
+	if (!proto_handlers(protocol) || !netproto(protocol))
+		return -EINVAL;
 
 	mutex_lock(&xfrm6_protocol_mutex);